alternans

Alternans refers to a beat-to-beat alternation in a physiological variable, such as amplitude, force, or duration. In cardiology, alternans most often describes alternating electrical or mechanical properties of the heart that occur at sufficiently high heart rates or in diseased states.

Electrical alternans is observed on the electrocardiogram as a beat-to-beat variation in QRS complex height or T-wave morphology, and can also refer to T-wave alternans, a periodic fluctuation in repolarization. A classic association is electrical alternans seen with large pericardial effusions and cardiac tamponade, where the heart swings within the fluid-filled pericardial sac. More generally, T-wave or QRS alternans may reflect instability in ventricular repolarization or excitation-contraction coupling and can be a marker of increased arrhythmic risk.

Mechanical alternans, or pulse alternans, involves alternating strong and weak arterial pulses or alternating stroke volumes. It is commonly seen in severe left ventricular dysfunction, ischemia, or acute heart failure and can indicate impaired ventricular performance.

At the cellular level, alternans can arise as action potential duration (APD) alternans and calcium transient alternans, often rate-dependent. These cellular phenomena can create spatial and temporal dispersion of repolarization, contributing to arrhythmia susceptibility.

Diagnosis typically relies on ECG monitoring, with specialized analyses for microvolt T-wave alternans, and imaging or hemodynamic assessment for mechanical alternans. Management focuses on treating underlying conditions (such as heart failure, ischemia, or tamponade), correcting electrolyte disturbances, and addressing arrhythmic risk with appropriate therapies and device interventions as indicated.